Aqueous emulsion polymerization process for the manufacturing of fluoropolymers

ABSTRACT

Emulsion polymerization of liquid fluorinated monomers which have a boiling point above 50° C. and low water-solubility is markedly more efficient if the monomer is pre-emulsified with a nontelogenic fluorinated emulsifier in water.

1. FIELD OF THE INVENTION

[0001] The present invention relates to the aqueous emulsionpolymerization of fluorinated monomers to produce fluoropolymers. Inparticular, the present invention relates to the aqueous emulsionpolymerization involving liquid fluorinated monomers which have aboiling point of at least 50° C.

2. BACKGROUND OF THE INVENTION

[0002] Aqueous emulsion polymerization is a well-known method for makingfluoropolymers, i.e. polymers of which the carbons in the backbone ofthe polymer have fluorine atoms attached to them. Generally, the amountof fluorination of the backbone of fluoropolymers will be at least 40%by weight, preferably at least 50% by weight. Fluoropolymers which havefully fluorinated backbones are called perfluoropolymers.

[0003] The fluoropolymers are generally produced by (co)polymerizationof gaseous fluorinated monomers, in particular fluorinated olefins suchas tetrafluoroethylene (TFE) hexafluoropropylene (HFP), vinyl fluoride,vinylidene fluoride (VDF) or chloro-trifluoroethylene (CTFE).

[0004] To obtain certain desired properties, it is known to copolymerisethe aforementioned gaseous fluorinated olefins with monomers such asfluoro alkenyl ethers which include fluoro vinyl ethers and fluoro allylethers. Fluoro alkenyl ethers that have a high boiling point of 50° C.or more have a low vapor pressure at typical polymerization temperaturesof 20° C. to 100° C. Additionally, they have an extremely low watersolubility. As a result it has proven difficult to incorporate theseliquid comonomers into fluoropolymers at larger levels as may be desiredfor certain applications. Incorporating large amounts of such comonomersis of particular value for producing elastomers that have a low Tg. Whenone wants to improve the amount incorporated of such monomers byincreasing the polymerization temperature, that also increases thelikelihood of undesired chain transfer reactions which impair thepossibility of obtaining high molecular weight. On the other hand,polymerization at low temperature reduces the rate of polymerizationmaking the process economically less attractive.

[0005] EP 219065 discloses an aqueous free radical emulsionpolymerization of perfluoro vinyl ethers of the formulaCF₂═CF—O—(CF₂CFXO)_(m)—R_(f) wherein X is F or CF₃, m is 1 to 5 andR_(f) is a C₁-C₆ perfluoroalkyl group. EP 219065 discloses the use ofcarboxylate terminated perfluoropolyether emulsifiers for conducting thepolymerization. However, in the examples of this EP-patent, fairly largeamounts of the emulsifier are used, typically about 10% by weight basedon the aqueous phase. Such high emulsifier concentrations usually leadto a so-called micro-emulsion polymerization which typically results inpolymer particles of considerably less than 100 nm.

[0006] EP 250767 discloses an aqueous free radical emulsionpolymerization involving for example perfluoroalkyl perfluorovinyl ethermonomers by using a microemulsion of a perfluoropolyether with polyetherbased emulsifiers. This microemulsion is taught to have droplet sizes ofless than 200 nm. The microemulsion is added to the polymerizationmedium at the beginning of the polymerization. The resulting particlesizes of the polymer particles obtained is very small, e.g. 40 nm, whichis evidence that the polymerization occurs via microemulsionpolymerization. Thus, the number of polymer particles per liter isincreased by an order of magnitude. Therefore, the polymerization rateis considerably increased. However, EP 250767 does not describepre-emulsification of liquid polymerizable fluorinated monomers likeperfluoroalkyl perfluorovinyl ether monomers.

[0007] EP 816397 discloses the use of similar microemulsions ofperfluoropolyethers with polyether based emulsifiers for the aqueousemulsion polymerization of vinylidene fluoride (VDF). As disclosed, theobtained latex particles have a very small size (apparently well below100 nm) and likewise an increase of the polymerization rate is found.The polymerization here therefore evidently also occurs viamicroemulsion polymerization.

[0008] U.S. Pat. No. 4,864,006 also discloses the use ofperfluoropolyethers microemulsified in perfluoroether-based emulsifiersat the aqueous emulsion polymerization of fluorinated monomers. All ofthese processes lead to microemulsion polymerizations characterized byan increased polymerization rate due to the observed small particlesize, considerably lower than 100 nm.

[0009] The perfluoropolyether emulsifiers have the disadvantage thatthey have high boiling points and are difficult to remove from thepolymer resin. Residual emulsifiers can have an adverse effect on theprocessing properties and the properties of the final product, forexample lead to discoloration during work-up or processing, or tobleed-out during long-term use of the final product.

[0010] U.S. Pat. No. 5,608,022 teaches the aqueous emulsionpolymerization of sulfonyl fluoride or carboxylic acid ester containingfluorinated vinyl ethers wherein the so-called functionalized vinylether monomer is pre-emulsified. According to Example 1 of thisUS-patent, the droplet size of the monomer droplets in the emulsion is550 nm.

[0011] It would be desirable to find an improved aqueous emulsionpolymerization process that allows for effective incorporation of largeamounts of highly boiling fluorinated monomers. Desirably, thepolymerization reaction should proceed at a high speed allowing for amore cost effective production of such fluoropolymers. The processdesirably may be run using low emulsifier levels and using emulsifiersthat can be readily removed from the resulting polymerization product.

3. SUMMARY OF THE INVENTION

[0012] The present invention provides a method of preparing afluorinated polymer including repeating units derived from a liquidfluorinated monomer having a boiling point of at least 50° C. andselected from the group consisting of fluorinated olefins, fluorinatedallyl ethers and fluorinated vinyl ethers that do not containhydrolysable groups that upon hydrolysation yield ionic groups, themethod comprising the steps of:

[0013] pre-emulsifying said liquid fluorinated monomer in water with theaid of a fluorinated emulsifier to obtain an aqueous emulsion of saidfluorinated monomer; and

[0014] polymerizing a thus obtained emulsified fluorinated monomer.

[0015] Preferably the pre-emulsified liquid fluorinated monomer iscopolymerized with one or more gaseous fluorinated monomers.

[0016] By the term “liquid fluorinated monomer” is meant that themonomer is generally present as a liquid at ambient conditions oftemperature and pressure, i.e. at a temperature of 20° C. and a pressureof 1 atm. By the term “pre-emulsified” in connection with the presentinvention is meant that the fluorinated monomer is emulsified in waterwith the aid of the fluorinated emulsifier prior to polymerization ofthe liquid fluorinated monomer. The term aqueous emulsion is to beunderstood as a liquid emulsified in water that generally has a milkyappearance generally having a settling time of at least 1 hour. Suchsettling time is generally achieved using a fluorinated emulsifier(preferably a non-telogenic emulsifier) other than a fluorinatedpolyether emulsifier. By the term “boiling point” in connection with thepresent invention is meant a boiling point under ambient conditions,i.e. at a pressure of about 1 atm. By the term “gaseous” in connectionwith the present invention is meant that the respective compounds arepresent as a gas under ambient conditions of temperature and pressure,i.e. at a temperature of about 20° C. and a pressure of about 1 atm. Thefluorinated vinyl ether should be free of hydrolysable groups that uponhydrolysation yield ionic groups, in particular the fluorinated vinylether should not contain hydrolysable groups such as esters and SO₂F.The fluorinated vinyl ether may contain substituents like chlorine,bromine and iodine, fluorinated alkyl groups, fluorinated alkoxy groupsand fluorinated polyether functions.

[0017] It was found that when the liquid fluorinated monomer wasemulsified prior to its copolymerization with gaseous fluorinatedmonomers, the polymerization time could be reduced and high levels ofthe liquid fluorinated monomer could be readily copolymerized. Inparticular, the process of the invention allows for the production offluoropolymers having at least 1% by weight of repeating units derivedfrom the pre-emulsified fluorinated monomer, preferably at least 5% byweight and more preferably at least 10% by weight. The process of theinvention also makes it possible to produce homopolymers of thepre-emulsified liquid fluorinated monomer. Also, commonly employedfluorinated emulsifiers that can be readily removed subsequent topolymerization can be used and they can be used in fairly low levels,typically of about 0.1% by weight to 1% by weight relative to the amountof water.

4. DETAILED DESCRIPTION OF THE INVENTION

[0018] In accordance with the process of the present invention, thefluorinated monomer is emulsified in water with the aid of a fluorinatedemulsifier prior to its polymerization or copolymerization with thegaseous fluorinated monomers. The pre-emulsification of the liquidfluorinated monomer preferably results in an emulsion having monomerdroplets of an average diameter of 1 μm or more, typically it isexpected in the range of 1 to 20 μm. The aqueous emulsion shouldpreferably have a pot life (settling time) of at least 1 hour, morepreferably at least 3 hours. The pot life or settling time is defined asthe time required for 10% by weight of the monomer droplets to settle orseparate out of the aqueous emulsion.

[0019] Aqueous emulsions of the liquid fluorinated monomer canconveniently be obtained by suitable emulsification equipment such asfor example a dispersing device such as Ultra-Turrax. The stirring ratesshould be sufficiently high to achieve the desired degree ofemulsification and stability. Generally, stirring rates of 24 000 rpm ormore can be employed. According to indication of the supplier of theUltra-Turrax device, the resulting emulsions would have an averagedroplet size of 1 to 10 μm. Air is preferably excluded during theemulsification.

[0020] Fluorinated emulsifiers for preparing the aqueous emulsion of theliquid fluorinated monomer are preferably non-telogenic emulsifiers.Examples of fluorinated emulsifiers include salts, in particularammonium salts of linear or branched perfluoro alkyl carboxylic andsulphonic acids having 6 to 11 carbon atoms in the alkyl chain. It wasfound that salts of branched perfluoroalkyl carboxylic and sulphonicacids are more effective than their linear counter parts. Specificexamples include perfluorooctanoic acid ammonium salt (APFOA) andC₈F₁₇SO₃Li. Commercially available fluorinated emulsifiers that can beused include C₈F₁₇SO₃Li available as FT 208 from Bayer AG, andC₈F₁₇SO₂N(C₂H₅)CH₂COOK available from 3M as FC 128.

[0021] APFOA is the preferred emulsifier as it can be more readilyremoved from the polymerization product at the end of polymerization.Typically, APFOA is very volatile and decomposes at temperatures of 180°C. or more leading to volatile products. Therefore, APFOA type ofemulsifiers can be easily removed during work-up of the fluoropolymers.

[0022] The pot life of the aqueous emulsion of the liquid fluorinatedmonomer can be increased by increasing the pH. For example, with APFOA,the pot life can be increased to 50 hours or more at a pH of 7 or more.Typically, the pH of the aqueous emulsion can be increased by addingammonia or alkali metal hydroxides.

[0023] The amount of fluorinated emulsifier used to emulsify the liquidfluorinated monomer is generally between 0.1 and 1% by weight based onthe weight of the water phase. Although higher amounts of emulsifier canbe used, they will not necessarily lead to a significant increased potlife of the aqueous emulsion.

[0024] The aqueous emulsion of the liquid fluorinated monomer mayinclude a mixture of liquid fluorinated monomers and may furthercomprise other monomers such as other poorly water soluble and liquidmonomers. Additional emulsifier may be added and if added, they arepreferably identical to the emulsifier(s) used to pre-emulsify theliquid fluorinated monomer. Advantageously, APFOA is used in theemulsification of the liquid fluorinated monomer and in the subsequentaqueous emulsion polymerization.

[0025] The pre-emulsification of the liquid fluorinated monomer mayproceed in a separate vessel from the polymerization vessel, in thepolymerization vessel itself prior to commencing the polymerization orin the monomer supply line to the polymerization vessel.

[0026] The aqueous emulsion polymerization can be carried outcontinuously in which, for example, the aqueous emulsion of the liquidfluorinated monomer and other monomers, water, optionally furtheremulsifiers, buffers and catalysts are fed continuously to a stirredreactor under optimum pressure and temperature conditions while theresulting emulsion or suspension is removed continuously. An alternativetechnique is batch or semibatch polymerization by feeding theingredients into a stirred reactor and allowing them to react at a settemperature for a specified length of time or by charging ingredientsinto the reactor and feeding the monomers and emulsified liquidfluorinated monomer into the reactor to maintain a constant pressureuntil a desired amount of polymer is formed. The polymerization can becarried out in a standard or conventional vessel used for emulsionpolymerization of gaseous fluorinated monomers.

[0027] For the free-radical polymerization use may be made of anysuitable initiator or any suitable initiator system, for exampleammonium persulfate (APS), or of redox systems, such as APS/bisulfiteand potassium permanganate. If oil-soluble initiators are used in thepolymerization, it is generally preferred for these to be mixed with theaqueous emulsion of the liquid fluorinated monomer. For the purposes ofthe present invention, oil-soluble initiators are those which have no,or only insufficient, solubility in water. Examples of oil-solubleinitiators are substituted dibenzoyl peroxides and cumenehydroperoxides, in particular bisperfluoropropionyl peroxide.

[0028] The polymerization systems may comprise auxiliaries, such asbuffers and, if desired, emulsifiers, complex-formers or chain-transferagents. The polymerization temperature may be from 10 to 100° C.Polymerization pressures may be from 3 to 30 bar.

[0029] The achievable solids content of the polymer dispersion istypically from 10 to 35% without any major coagulation. Surprisingly,the resultant dispersions are highly stable despite the observedunusually large particle diameters of up to 400 nm. The particle size ofpolytetrafluoroethylene (PTFE) born via standard emulsion polymerizationdo not exceed 250 nm, larger particle size result in coagulation of thePTFE at polymerization conditions; the particle size of fluoroelastomersand thermoplasts born in standard emulsion polymerization usually do notexceed 200 nm. The large particle size observed is believed to be due tothe emulsion polymerization occurring partially via a so-calledmonomer-droplet polymerization.

[0030] The production of fluoropolymers via aqueous emulsionpolymerization generally produces considerable amounts of industrialwastewater as a consequence of necessary washing processes during theisolation of the resins from the polymer dispersion. The wastewatergenerally comprises about 90% of the emulsifier used. Since theemulsifier makes a considerable contribution to production costs, it ispreferably recovered and recycled from the wastewater, using a processas disclosed in German Patent Application 199 53 285.0.

[0031] The aqueous emulsion polymerization process of the presentinvention can be used for the polymerization of liquid fluorinatedmonomers that have a boiling point of at least 50° C. and that areselected from the group consisting of fluorinated olefins, fluorinatedallyl ethers and fluorinated vinyl ethers that do not containhydrolysable groups capable of yielding ionic groups when hydrolysed.The liquid fluorinated monomer may be fully or partially fluorinated,i.e. one or more fluorine atoms may be replaced by iodine, bromine,chlorine or hydrogen. However, when the liquid fluorinated monomer ispartially fluorinated, the molar ratio of fluorine to non-fluorine atomsor groups should generally be at least 4. Examples of liquidfluoroolefins include partially and fully fluorinated olefins having aboiling point of at least 50° C. Specific examples includeperfluoroalkyl vinyl monomers having 6 or more carbons in theperfluoroalkyl chain such as C₆F₁₃—CH═CH₂.

[0032] The process of the invention may further be used to polymerizeliquid fluorinated allyl ethers which are fluorine compounds that haveat least one ether bridge and one terminal double bond, with the oxygenin β-position with respect to the double bond. Examples of fluorinatedallyl ethers include those that are fully as well as those that arepartially fluorinated. Examples of fluorinated allyl ethers are thosethat correspond to the formula:

R_(f)—O—CF₂—CF═CF₂  (I)

[0033] wherein R_(f) represents a linear or branched perfluoroalkylgroup which may contain one or more ether linkages. A specific exampleof a liquid fluorinated allyl ether according to formula (I) isCF₃—O—(CF₂)₂—O—CF₂—CF═CF₂ (boiling point 67° C.).

[0034] Preferably, the liquid fluorinated monomer for use in theinvention is a fluorinated vinyl ether. Generally, the fluorinated vinylether is a perfluoro vinyl ether. Typically, the fluorinated vinyl etheris a perfluorinated vinyl ether corresponding to formula (II):

CF₂═CFO(R_(f)O)_(n)(R′_(f)O)_(m)R″_(f)  (II)

[0035] wherein R_(f) and R′_(f) are different linear or branchedperfluoroalkylene groups of 2-6 carbon atoms, m and n are independently0-10 and the sum of n and m is at least 1, and R″_(f) is aperfluoroalkyl group of 1-6 carbon atoms.

[0036] Examples of fluoro vinyl ethers of formula (II) include compoundsof the formula (III):

CF₂═CFO[CF₂CF₂CFZO]_(n)R″_(f)  (III)

[0037] where R″_(f) is a perfluoroalkyl group having 1-6 carbon atoms,n=1-5, and Z=F or CF₃. Examples of compounds of formula (III) includethose in which R″_(f) is C₃F₇, n=1, Z=F or CF₃ andCF₃—O—CF(CF₃)—CF₂—CF₂—O—CF═CF₂ (boiling point 85° C.). Further usefulfluorinated vinyl ethers included in formula (II) correspond to theformula (IV):

CF₂═CF—O—(CF₂CFXO)_(m)—R_(f)  (IV)

[0038] in which m represents an integer of 1 to 5, X is F or CF₃ andR_(f) is a C₁-C₅ perfluoroalkyl group. Examples of monomers of formula(IV) include those where X is CF₃ and R_(f) is perfluoro n-propyl andwherein m is 1 (boiling point of 103° C.) or m is 2 (boiling point of160° C.).

[0039] Additional perfluoro vinyl ether monomers useful in the inventioninclude compounds of the formula

CF₂═CFO[(CF₂CFCF₃O)_(n)(CF₂CF₂CF₂O)_(m)(CF₂)_(p)]C_(x)F_(2x+1)  (V)

[0040] where m and n=1-10, p=0-3, and x=1-5. Preferred members of thisclass include compounds where n=l, m=1, and x=1.

[0041] Still further examples of perfluoro vinyl ethers include etherscorresponding to formula (VI):

CF₂═CFOCF₂CF(CF₃)—O—(CF₂O)_(m)C_(n)F_(2n+1)  (VI)

[0042] where n=1-5, m=1-3, and where, preferably, n=1.

[0043] Copolymerization of perfluorinated vinyl ether monomers such asset forth above with gaseous fluorinated monomers such as TFE, resultsin polymers with considerably improved properties. When incorporated atlow levels, typically less than 10% by weight, fluorinatedthermoplastics with improved properties can be obtained.Fluoroelastomers with desirable properties can be produced bycopolymerizing substantial amounts of fluorovinyl ethers with gaseousfluorinated monomers. In particular the low-temperature properties ofthe final product can be improved by incorporating high levels of theliquid fluorinated vinyl ether monomer into the fluoropolymer.Typically, the amount of repeating units derived from such fluorovinylethers, for example those according to formula (II) above, will bebetween 15 mol % and 50 mol %. Such fluoropolymers will have a glasstransition temperature (Tg) that is, for example, as low as −40° C.

[0044] Especially useful fluoro vinyl ethers for making fluoroelastomerswith a low Tg are those that have linear perfluoro groups attached tothe vinyl of the fluoro vinyl ether, such as monomers according toformula (III) in which X is F. A highly preferred monomer for makingfluoroelastomer is CF₃—O—(CF₂)₃—O—CF═CF₂ (boiling point of 64° C.).

[0045] The preparation of vinyl/allyl ethers is generally expensive andhas been described in DE-A-22 15 401. The process of the inventionallows for the effective incorporation of these monomers intofluoropolymers without substantial waste of these expensive monomers.

[0046] To manufacture fluoropolymers and preferably perfluoropolymers bythe process of the present invention, the liquid fluorinated monomersare generally copolymerized with gaseous fluorinated monomers, inparticular gaseous fluorinated olefins. Examples of gaseous fluorinatedmonomers for copolymerization with the liquid fluorinated monomersinclude tetrafluoroethylene (TFE) hexafluoropropylene (HFP), vinylfluoride, vinylidene fluoride (VDF), chlorotrifluoroethylene (CTFE) andmixtures thereof. Additionally, the copolymerization may involvenon-fluorinated gaseous monomers such as ethylene and propylene.

[0047] Generally, the amount of repeating units derived from the gaseousfluorinated monomers in the fluoropolymer will be at least 20% byweight, preferably at least 40% by weight. A typical amount of repeatingunits derived from gaseous fluorinated monomers is between 50% by weightand 95% by weight.

[0048] Various modifications and alterations of this invention willbecome apparent to those skilled in the art without departing from thescope and spirit of the present invention, and it should be understoodthat this invention is not to be unduly limited to the illustrativeembodiments set forth herein.

[0049] The invention will now be further illustrated with reference tothe following examples without however intending to limit the inventionthereoto.

EXAMPLES

[0050] List of Abbreviations Used in the Examples:

[0051] APFOA Ammonium salt of perfluorooctanoic acid

[0052] APS Ammonium persulfate

[0053] TFE Tetrafluoroethylene

[0054] VDF Vinylidene fluoride

[0055] PF-3: CF₃—O—(CF₂)₃—O—CF═CF₂ boiling point 64° C.

[0056] PF-4: CF₃—O—CF(CF₃)—CF₂CF₂—O—CF═CF₂ boiling point 85° C.

[0057] PF-5: CF₃—O—CF₂—CF₂—O—CF₂—CF═CF₂ boiling point 67° C.

[0058] PPVE-2: CF₂═CF—O—CF₂—CF(CF₃)—O—CF₂—CF₂—CF₃ boiling point 103° C.

[0059] Test Methods:

[0060] Determination of physical parameters: the comonomer content ofthe copolymer is measured using ¹H NMR and ¹⁹F NMR. Acetone and,respectively, hexafluorobenzene are used as solvent. The glasstransition temperature is determined by differential thermal analysis(DSC), and the particle size is determined by elastic light scattering.

Comparative Example 1

[0061] A 4 l vessel which has been equipped with an impeller agitator ischarged with 2.8 l of deionized water which comprises 5 g of APFOA and 9g of K₂HPO₄ and heated to 70° C., and atmospheric oxygen is removed byalternating nitrogen-flushing with evacuation. 454 g of PVVE-2,deaerated by nitrogen-flushing, are then charged to the vessel and thepressure is brought to 9.0 bar using 48 g of TFE and 71 g of VDF. Thestirring rate is 320 rpm. The polymerization is initiated by rapidlyfeeding 3 g of APS dissolved in 15 ml of water. The polymerization ismaintained at constant pressure and temperature for 2 hours,continuously feeding 434 g of PPVE-2. The pressure is held constant byintroducing TFE and VDF in a ratio of 1:1.9 by weight. A micromotionmeasurement device is used to measure the take-up rate of the gaseousmonomers. After the desired polymerization time, the gaseous monomersare drawn off. The reactor is cooled to room temperature and itscontents discharged. The polymerization is terminated by ceasing monomersupply and aerating the vessel. The discharged vessel contents divideinto two layers. The lower layer is unconverted PPVE-2 (762 g). Theupper layer comprises 332 g of copolymer composed of 31 mol % of TFE, 68mol % of VDF and 0.7 mol % of PPVE-2. Only 14.5% of the PPVE-2 used wasincorporated into the polymer.

Comparative Examples 2 to 5

[0062] In studies on a variety of copolymerization systems, the reactionconditions and procedure were varied from Comparative Example 1 only inrespect of the conditions listed in Table 1a. The findings are given inTable 1b.

Examples 1 to 6

[0063] These are examples according to the invention in which the liquidfluorinated monomer (liquid monomer) was pre-emulsified, i.e. emulsifiedprior to its polymerization. 500 g of PPVE-2 and 780 g of deionizedwater which comprises 5 g of APFOA (tradename FX 1006, supplied by 3M)are charged to a glass container which has been equipped with adispersing device (“Ultra-Turrax”, IKA Turrax T 25 S 25 N 18 G). Thecontents are flushed with nitrogen.

[0064] The pH is adjusted to 7.0 using ammonia. The 2-phase mixture isstirred for 30 seconds at 24,000 rpm. This gives a milky emulsion with apot life ≧3 h. The pre-emulsified liquid fluorinated monomer is meteredin and also, where appropriate, used as a subsequent feed.

[0065] The same 4 l vessel is utilized, with the same stirringconditions. The reaction conditions are given in Table 2a and thefindings in Table 2b. The pre-emulsification utilizes the amount ofemulsifier given in the table, and this is the total amount utilized inthe polymerization. The pre-emulsified monomer is fed continuously at aconstant rate over the polymerization time given in Table 2a. No lowerphase is observed in any of the examples shown. The liquid monomer isincorporated practically quantitatively into the polymer. The glasstransition temperature of the copolymer prepared according to theinvention is also given in Table 2b.

Example 6 (Homopolymerization of PPVE-2)

[0066] 500 g of PPVE-2 are pre-emulsified under nitrogen with 740 g ofwater in which 30 g of FT 208 have been dissolved as fluorinatedemulsifier. The homopolymerization is carried out in a 4 l vessel inwhich there are 1460 g of water.

[0067] The vessel contents are freed from air by repeated evacuation andnitrogen-flushing, heated to 70° C. and brought to 1.2 bar usingnitrogen. The pre-emulsified liquid fluorinated monomer is then pumpedinto the vessel, and the polymerization initiated by feeding 4 g of APS,dissolved in 150 g of water. After 5 hours the polymerization isterminated by cooling the contents of the vessel to room temperature.This gives 2895 g of a milky dispersion with a solids content of 8.5%,corresponding to 240 g of polymer. NMR analysis shows that the polymeris a PPVE-2 homopolymer. TABLE 1a Reaction conditions for thecopolymerization systems without pre- emulsification: Polymerizationtemperature: 70° C. Polymerization pressure: 9 bar Initiator: APSEmulsifier: APFOA Buffer: 9 g of K₂HPO₄ Amount of liquid Weight ratiomonomer between gaseous [g] monomers Reaction Comparative Liquid InitialSubsequent Gaseous Initial Subsequent APS APFOA time Ex. No. monomercharge Feed comonomers charge feed [g] [g] [h] 2 PPVE-2 500 — TFE 139 g107 g 0.05 13.5 0.6 3 PF-3 266 200 TFE/VDF 1:1.4 1:1.95 9.0 9.0 1.55 4PF-4 346 386 TFE/VDF 1:1.4 1:1.95 9.0 9.0 3.5 5*) PF-5 133 218 TFE/VDF 1:2.35 1:1.66 4.5 12.0**) 1.3

[0068] TABLE 1b Experimental findings Compara- Amount of Amount ofComposition of tive lower Percentage of copolymer copolymer in Examplephase unconverted in upper mol % No. [g] liquid monomer phaseTFE/VDF/MF*) 2 397 79 209 not determined 3 202 43 452 28/59/13 4 590 80434 30/62/7  5 192 55 740  7/81/12

[0069] TABLE 2a Reaction conditions for the copolymerization systemswith pre- emulsification of the liquid fluorinated monomer (PPVE-2):Polymerization pressure: 9 bar Initiator: APS Emulsifier: APFOA Buffer:9 g of K₂HPO₄ Amount of Weight ratio liquid Between monomer gaseous [g]monomers React. React. Ex. Liquid Initial Subs. Gaseous Initial Subs.APFOA APS temp. time No. monomer charge Feed comonomers charge Feed [g][g] [° C.] [h] 1 PPVE-2 300 490 TFE/VDF 1:1.5  1:2.0  9*) 0.5 70 2.5 2PPVE-2 500 — TFE 113 g 141 g 4.5 1 70 1.0 3 PF-3 290 290 TFE/VDF 1:1.421:1.85 9   9 60 3.0 4 PF-4 250 330 TFE/VDF 1:1.42 1:2.2  9   60 2.9 5PF-5 133 217 TFE/VDF 1:2.4  1:1.66 12*)  4.5 60 1.7

[0070] TABLE 2b Experimental findings Composition of Glass Amount ofSolids copolymer transition Ex. dispersion content (mol %) temperatureNo. [kg] [%] TFE/VDF/MF*) [° C.] 1 4.0 24.9 16/57/27 −30 2 3.55 13.080/0/20  −6 3 3.86 24.3 19/57/24 −40 4 3.83 24.8 24/55/22 −30 5 3.8527.1 21/68/10 −35

1. Method of preparing a fluorinated polymer including repeating unitsderived from a liquid fluorinated monomer that has a boiling point of atleast 50° C. and is selected from the group consisting of fluorinatedolefins, fluorinated allyl ethers and fluorinated vinyl ethers that donot contain hydrolysable groups that upon hydrolysation yield ionicgroups, the method comprising the steps of: pre-emulsifying said liquidfluorinated monomer in water with the aid of a fluorinated emulsifier toobtain an aqueous emulsion of said fluorinated monomer; and polymerizinga thus obtained emulsified liquid fluorinated monomer.
 2. Methodaccording to claim 1 wherein said fluorinated vinyl ethers consist ofcarbon and fluorine atoms and are optionally substituted with asubstituent selected from the group consisting of hydrogen, chlorine,bromine and iodine.
 3. Method according to claim 1 wherein saidemulsified liquid fluorinated monomer is copolymerized with one or moregaseous fluorinated monomers.
 4. Method according to claim 1 whereinsaid fluorinated emulsifier is a non-telogenic emulsifier.
 5. Methodaccording to claim 3 wherein the gaseous fluorinated monomer is selectedfrom the group consisting of tetrafluoroethylene, hexafluoropropylene,vinyl fluoride, vinylidene fluoride, chlorotrifluoroethylene andmixtures thereof.
 6. Method according to claim 3 wherein saidpreemulsified liquid fluorinated monomer is further copolymerized withone or more non-fluorinated olefinic monomer.
 7. Method according toclaim 6 wherein said non-fluorinated olefinic monomer is selected fromthe group consisting of ethylene and propylene.
 8. Method according toclaim 1 wherein the fluoropolymer prepared has at least 1% by weight ofrepeating units derived from said liquid fluorinated monomer.
 9. Methodaccording to claim 1 wherein the fluoropolymer prepared has at least 40%by weight of repeating units derived from said gaseous fluorinatedmonomers.
 10. Method according to claim 1 wherein said aqueous emulsionhas a settling time of at least 1 hour.
 11. Method according to claim 1wherein said liquid fluorinated monomer is a perfluorinated vinyl ethercorresponding to the formula: CF₂═CFO(R_(f)O)_(n)(R′_(f)O)_(m)R″_(f)wherein R_(f) and R′_(f) are different linear or branchedperfluoroalkylene groups of 2-6 carbon atoms, m and n are independently0-10 and the sum of n and m is at least 1, and R″_(f) is aperfluoroalkyl group of 1-6 carbon atoms.
 12. Method according to claim11 wherein said perfluorinated vinyl ether corresponds to the formula:CF₂═CF—O—(CF₂)₃—O—R_(f)″ wherein R_(f)″ is a perfluoroalkyl group having1 to 6 carbon atoms.